There is a need especially in telecommunications for a switch which can switch both ac and dc currents, can take current surges of either polarity, can interrupt dc loop currents, and has linear through-the-origin current voltage characteristics.
To satisfy this need, there have been proposed, hitherto, switches which include field-effect transistors whose conduction state has been controlled by optical illumination. Typically, such switches include a field-effect transistor biased to be normally off, which is turned on when light is incident on a photovoltaic element connected to provide an appropriate turn-on bias in the gate-bias circuit of the transistor. It has also been recognized that for increasing the current-handling capacity of the transistor, as for example to adapt it for surge protection, it is advantageous to include in shunt with the field-effect transistor a thyristor which is truned on when current through the transistor exceeds a threshold value, whereby enough gate current is applied to the thyristor to turn it on. Once turned on, the thyristor can serve as a second main current path through the switch, permitting use in the switch of a transistor of lower current-handling capacity than is to be handled by the switch.
However, it is a characteristic of thyristors that once turned on they tend to continue in this state so long as a minimum holding current is maintained through the thyristor even though the gate triggering current has been removed. In many possible applications of a solid-state switch of the kind involved, in normal operation there is continuously being applied across the switch terminals operating voltages of a magnitude sufficient to maintain a current through the switch larger than the minimum holding current if there is no other path available for the current, as when the transistor path is open because the transistors are properly in a nonconducting state as the result of the absence of an "ON" control signal. In this case, a thyristor, once triggered on by a voltage surge in performing its protective role, tends to continue to conduct even though the surge subsides and the control signal is off. This action is described as latching on. This tendency makes for unreliable operation of the switch.